You have to factor in efficiency. 80%+ is expected with this PSU, so when it draws 800W, it uses ~650W for the actual system. The PSU is safe till it draws ~900W or so. Because 750W is not factoring in efficiency.

Also, I don't know if anyone would know or not, but later this week I will be removing the stock fans from my 580s and using Waterblocks, since my cooling loop is already running on the system will this decrease power consumption?